Don't focus on earnings. P/Es are useless in a harsh recession or depression with financial collapse, because earnings will obviously be at a trough or negative for many stocks in the index.
You should look at more robust valuation measures such as price to sales, price to book, market capitalization as a % of GDP, and dividend yields. Be aware of likely cuts/falls in those measures as well.
IMO stocks will need to go to pretty fat dividend yields before a bottom is put in. Work out the implied S&P price if dividends were to yield a conservative 6%, for example. And it's quite possible blue chip yields go to 10 or 12%, as they did in the UK in late 1974.